Lithium-sulfur batteries are a secondary battery using sulfur-based compounds having sulfur-sulfur bonds as a cathode active material, and using alkali metals such as lithium, or carbon-based materials capable of intercalation and deintercalation of metal ions such as lithium ions as an anode active material. Electric energy is stored and generated using an oxidation-reduction reaction reducing an oxidation number of sulfur as sulfur-sulfur bonds are broken during discharge, a reduction reaction, and forming sulfur-sulfur bonds again as an oxidation number of the sulfur increases during charge, an oxidation reaction.
The lithium-sulfur battery is promising in terms of energy density since energy density is 3830 mAh/g when using lithium metal used as an anode active material, and energy density is 1675 mAh/g when using sulfur (S8) used as a cathode active material. In addition, sulfur-based materials used as a cathode active material have an advantage in that they are low-priced and environmental-friendly.
However, sulfur is close to a nonconductor having electric conductivity of 5×10−3° S/cm, and has a problem in that transfer of electrons generated from an electrochemical reaction is difficult. Accordingly, an electric conductor such as carbon capable of providing a smooth electrochemical reaction site needs to be used. Herein, when the conductor and the sulfur are simply mixed, the sulfur leaks to an electrolyte during an oxidation-reduction reaction, and then the leaked sulfur deteriorates battery life. When the conductor and the sulfur are simply mixed, and a suitable liquid electrolyte is not selected, elution of lithium polysulfide, a reduced material of sulfur, occurs leading to a problem of making participation in the electrochemical reaction impossible.
Consequently, there have been demands for improving a quality of carbon and sulfur mixture so that elution of sulfur to an electrolyte is reduced, and electric conductivity of an electrode including sulfur increases.